This invention relates to an apparatus and method for protecting superconducting magnetic energy storage systems from damage upon a sudden loss of superconductivity causing a rapid dissipation of the stored energy, and more particularly, to the use of suitable thermal capacitance material coupled to a superconducting coil system for absorbing heat generated in the system in the event the coil suddenly becomes nonsuperconducting due to a malfunction or the like while storing a substantial amount of energy.
An informal report by the Los Alamos Scientific Laboratory in September 1979 entitled "1-GWh Diurnal Load-Leveling Superconducting Magnetic Energy Storage System Reference Design", designated "LA-7885-MS, Vol. 1", outlined a design for a load-leveling system for electric utilities. That system increases the overall efficiency of commercial power plants by storing energy in a large, underground superconducting magnetic energy storage coil during periods of low demand, and using the stored energy during peak periods. With such a system, existing fossil fuel generators operate at a higher average percentage of their rated output, and the capacity of the generating system does not have to be increased just to meet the peak demand, which may last only a short time each day. The growth of the nuclear power industry has increased interest in load-leveling systems because the power output of nuclear fission power plants is not easily reduced during periods of low demand. Without a load-leveling system, a substantial amount of power is generated and lost, which is very inefficient.
Load-leveling systems are already in use n this country. A load-leveling system near Ludington, Michigan uses power generated during periods of low demand to pump water from Lake Michigan to a holding area above the lake water level. When demand approaches or exceeds the operating capacity of the generating system, the stored water is released through turbine generators, which produce additional electrical power. Thus, during slack periods electrical energy is converted into kinetic energy by the pumps which elevate the water. A substantial portion of that energy is stored as potential energy in the elevated water in the holding area, and is again converted into kinetic energy when the water is released. This kinetic energy is converted into electrical energy by the generators. While this system increases the overall utility of the generating system and lowers average cost per kilowatt-hour, it is not efficient itself because the energy is converted several times at less than 100 percent efficiency each time. A superconducting magnetic energy storage (SMES) system is particularly suitable for load-leveling because the electrical energy remains as electromagnetic energy and is not converted into mechanical energy. Such a system may have a high efficiency in the range of 80 percent to 95 percent.
The power storage capability of an SMES system must be fairly large to be cost effective, because the cost per unit of energy is generally inversely proportional to the cube root of the maximum energy which may be stored in the system. A power capacity of about one gigawatt-hour (1 GWh) is suggested in the Los Alamos report. The proposed superconducting coil would have about 4,280 turns, an average radius of about 66 m, a height of about 44 m, and an operating current at full charge of about 50 kA. Various other coil designs have been suggested by others.
The Los Alamos superconducting coil would be housed beneath the ground in a vacuum, and the current carrying conductor would be cooled by liquid helium or the like. Filament protection apparatus might be provided which protects the coil if it becomes nonsuperconductive and begins conducting in a normal manner with relatively high resistivity for a very brief period of time. Such state changes may occur when the coil starts to charge or discharge.
A generally known approach to filament protection involves embedding or lacing superconductor filaments with high purity aluminum or the like. The aluminum has a higher resistance than the filaments when the latter are in a superconductive state, and a lower resistance than the filaments when they are in a normal state. If the coil becomes nonsuperconductive, the current transfers to the aluminum, which dissipates the heat generated by the current. When the filaments become superconductive again, the current returns to the filaments, which then have a lower resistance. Such state changes are normal, and do not relate to this invention.
Abnormal state changes in which the coil is nonsuperconductive for more than a very brief period may generate more heat than the protection apparatus can absorb. The apparatus of this invention protects the coil under such circumstances. Such abnormal changes are likely to occur at least once over a period of several years, for although the system is designed for completely stable and reliable operation, at least a portion of the coil could become nonsuperconductive for an abnormal length of time due to a sudden loss of coolant, loss of vacuum or other malfunction. A superconducting coil carrying current in the range of 50 kA generates a large voltage drop if it suddenly becomes nonsuperconductive (normal), and the energy stored in the coil rapidly heats the normal region, generating tremendous heat in a short period of time. The coil must be protected from such over-heating and excessive voltage, or it may be severely damaged.
A common method of protecting a superconducting coil in the event of a malfunction is to divide the coil into small energy blocks by the use of multiple current leads, and provide a dump resistor and switch for each block. In the system suggested in the Los Alamos report, the number and size of the coil windings, and the large current density in the coil, make such an approach impractical because several hundred switches, resistors, and current lead pairs would be required, which would increase the capital and maintenance cost of the system intolerably. The switches would be located outside the superconductor at about room temperature, heat would propagate through the current leads, and additional energy would be required to cool the system.
Another method of protection is to permit the coil, energy storage system and surrounding supports for the storage system to absorb the heat generated by the stored energy. Since the outer support system, which may be bedrock or the like, is thermally isolated from the conductor, substantially all of the energy would be absorbed by the conductor and inner structure of the coil. To absorb such a substantial amount of heat, the area of the coil and surrounding apparatus would have to be increased dramatically to reduce the energy per unit mass. Such a solution would greatly increase the size of the system, and would be expensive and impractical.
The potentially destructive current produced by the sudden collapse of a magnetic field in a superconducting coil may be transferred to a secondary winding and external dumping circuit which is cooled by gaseous helium, as suggested in U.S. Pat. No. 3,458,763. The gaseous helium is produced from liquid helium normally used to cool the conductor, which is vaporized when the coil becomes nonsuperconducting. The gaseous helium is passed over a resistive load in the dumping circuit before it is released from the system. Such a system is impractical for the application contemplated in the Los Alamos report, however, because it would require substantial additional material at great expense. Thus, there is a need for an apparatus and method for economically protecting a superconducting coil from damage which may be caused by a rapid conversion of stored energy which may occur if the coil suddenly becomes nonsuperconducting for more than a very brief period.
Accordingly, one aspect of this invention is to provide a new and improved apparatus and method for protecting a superconducting coil from damage due to loss of its superconductive state and consequent conversion of stored energy.
Another aspect is to provide a practical new and improved method and apparatus for protecting a superconducting coil from damage due to a sudden dissipation of energy.